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enter image description hereI couldn't find a similar thread that had my issue. I've recently replaced the cassette and chain within a week of each other (chain then cassette) I have a 2x9 setup. The chain slips on the cassette in 8th, and it's not consistent such as one rotation of the wheel or a derailleur rotation. I made the mistake of not checking chain length when I put it on but I did adjust it down to what I think is the right length.

I've done everything I can think, the only thing left is removing one more link which might(?) solve the problem but there isn't much slack currently anyway...

It's a Subway II from Halfords (UK) and it's a daily commuter bike so it isn't dry rode only, it's about 15-30k a day 5-6 days a week

Any help would be great. Thankyou in advance!

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    Can you use edit to add a clear and well-lit photo of your chain area viewed from the right-hand side? Set the chain on the biggest cog at the rear, and the biggest chainring at the front. This is to see if your chain is too long.
    – Criggie
    Commented Jun 20 at 11:08
  • I'll do that when I get back, thankyou! Commented Jun 21 at 12:37

4 Answers 4

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If new parts are skipping in 8th but not 9th, i.e. the 2nd smallest but not the smallest sprocket, I'd suspect indexing. Maybe the new cassette sits very slightly differently, or the wheel does after putting it back in.

You can check the chain length now. If you cross chain to big/big the rear derailleur should be pretty much totally stretched forwards, but everything should turn. I suggest not doing this while riding in case the chain is too short. If there's a fair bit of slack in big/big, your chain is too long after all, but I'd expect a slack chain to be worse in the smallest sprocket (fewer teeth engaged), and also worse for smaller chainrings (more slack)

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    Note that well-used gears wear faster, and smaller gears wear faster, so peak wear could easily be on this sprocket on an old cassette, as well as skipping being more likely with less engagement. So the answer really is based around the use of new parts
    – Chris H
    Commented Jun 21 at 14:26
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It's just possible the new chain is faulty and has a stiff link i.e. a link that doesn't rotate as freely as it should about the pin. I had this happen to me a couple of years ago and the symptoms were very like what you describe including mostly happening on one rear cog.

It's very easy to check for this. Just rotate the pedals backwards fast and watch the derailleur. If there is a stiff link you'll see the derailleur twitch every time the stiff link passes through it. If the derailleur stays motionless then you've eliminated this as a possible cause.

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  • Intriguing, I had thought about running it back to check for a stiff link, a stiff link didn't even occur to me... Shall check soon! Commented Jun 21 at 12:40
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Are you positive it's not the chainring? Try shifting into other gears besides 8th and really honk up a hill, does it slip?

If your ring looks "sharky" that's another clue that that is your issue. Because chainrings are often aluminum they can wear out sooner than cogs.

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  • Tbh I honk it at every red light, I normally get up to 25mph, I cruise in 7th 80-90rpm unless I want to get up higher or draft a car, almost instantly it clicks/skips and I'm putting enough watts through that the whole chain can twitch sometimes and dismount itself from somewhere on the drivetrain.. But I'll give it a check after looking into the things other comments mention too! Commented Jun 21 at 12:42
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There are many possible causes of chain skipping. As already said by others, it could be indexing or a worn out chainring. Other considerations:

Make sure your B screw is set properly. Refer to Park Tool's rear derailleur adjustment guide to see how to adjust B screw (and while you're at it, you may want to read through the entire article to get more familiar with how rear derailleurs work and how to properly adjust them): https://www.parktool.com/en-us/blog/repair-help/rear-derailleur-adjustment

Make sure your derailleur hanger is not bent. To do this, stand directly behind your bike and make sure that the derailleur pulley wheels appear perfectly parallel to the cassette cogs. Some bikes have replacable hangers, but steel frames typically have integrated hangers that can be bent back into shape.

Your rear derailleur or your shift cable could be worn out, damaged, or dirty, causing the derailleur to not fully settle into its proper position when shifting from 7th to 8th cog. I am guessing this is not the cause since you are only having the problem in the one cog, but it's a possibility. You can use oil such as Dri-Slide Bike Aid to lubricate a rear derailleur's spring and pivot points, as well as the shift cable itself (to help the cable slide freely through the housing).

Regarding chain length: When at home (not actually riding the bike), shift into the smallest chainring and the smallest cog. If the rear derailleur is fully relaxed and there is slack in the chain then the chain is too long. If the rear derailleur is not quite fully relaxed then you're fine, even if it's almost fully relaxed. (by "relaxed" I mean "in the same position that it would be in if the chain was removed")

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  • I had the old model of the subway before this one and both (thankfully) have a replaceable derailleur hanger, I'll post some pictures in a bid to solve this as another comment asked for it and I'll get some of that too, it doesn't appear warped - that being said I've replaced it before and that was a absolute break rather than a twist, it was on the old bike though! Commented Jun 21 at 12:40

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